So-called “balanced detectors” are known components in optical receiver circuits and typically include a pair of photodiodes that each receives a corresponding portion of an optical signal. The photodiodes may be coupled to respective inputs of a transimpedance amplifier (TIA). Each input is coupled to the output of the TIA via resistor (“feedback resistor”), such that the output voltage of the TIA corresponds to a difference between the currents generated by the photodiodes by the received optical signal portions. The output voltage may then be further processed to determine the information carried by the optical signal.
A TIA, as described above, however, may have several disadvantages. In particular, the number of connections between the photodiodes and the TIA is relatively high. As a result, the design of the TIA and photodiodes may be complicated, and the size of the die on which the TIA, as well as the photodiodes, is provided may be increased. Moreover, each connection may be fed to either a differential stage or to a corresponding TIA. In either case, the TIA/balanced detector may consume excessive power.
Further, the feedback resistors coupled between the TIA inputs and the TIA output have an associated thermal noise, which may be observed in the output voltage of the TIA. Thus, since a relatively high number of inputs are provided, the thermal noise associated with the TIA is also relatively high.
Moreover, each photodiode may supply a dark current, which further increases noise on the output of the TIA.
In addition, a DC offset may be required for each input to the TIA, otherwise the dynamic range of the TIA may be reduced. As a result, additional biasing circuitry may be required. Further, the impedance of each input is preferably matched to each other, but such matching may be difficult to achieve at high frequencies.
Accordingly, there is a need for a TIA in combination with a balanced photodetector that does not suffer from the above-note shortcomings.